Lyotropic liquid crystallinity was examined for 8 linear amylose tris(n-octadecylcarbamate) (ATODC) samples ranging in the weight-average molar mass (M-w) from 25 to 1510 kg mol(-1) and 10 cyclic ATODC (cATODC) samples, the M-w of which varies between 36 and 191 kg mol(-1). Appreciable cholesteric patterns were not observed in the concentration range investigated. Diffraction data from small-angle X-ray scattering both for bulk polymer samples at 160-180 degrees C and concentrated solutions at 25 degrees C indicate that no characteristic peaks were found except for a broad diffraction reflecting the chain distance between the neighboring polymer molecules. The phase boundary concentrations for ATODC and cATODC in tetrahydrofuran and 2-octanone were determined at 25 degrees C as a function of M-w. The obtained phase diagram for ATODC was well fitted by a conventional scaled particle theory (SPT) for the wormlike spherocylinder as in the case of the other stiff polymer systems. On the other hand, when we assumed that cATODC chains form a rodlike structure in the nematic phase, the experimental data were successfully explained by a modified SPT, suggesting that conformation of cATODC in the nematic phase is different from that in dilute solution.